In the optical transmission system, Optical Add/Drop Unit Board (OAD) is often used to implement the add/drop multiplexing of optical signals, and the principle of optical path is shown in FIG. 1. The role of the OAD in the optical transmission system is responsible for the add/drop channel multiplex and demultiplex in one waveband, and A1, A2, A3 and A4 are the optical ports in the add channel of four signals with specific wavelengths of OAD, and the local services are uplinked to the system via these four optical ports to carry out transmission, and D1, D2, D3 and D4 are the optical ports in the drop channel of four signals with specific wavelengths of OAD, and the services transmitted in the system are downlinked to the local area via these four optical ports, and IN and OUT are the input and output optical ports of the OAD respectively. In practical network systems, it needs to detect the optical power of the signals at the optical ports A1, A2, A3, A4, D1, D2, D3 and D4 in the add and drop channels respectively. The function of service of the add/drop channel is implemented via one optical multiplexing and demultiplexing module fixed in the OAD, and the module is integrated with a positive intrinsic negative diode (PIN) inside. With the PIN diode, the optical signal through the eight optical ports in the add and drop channels are converted into a current signal which is generally very weak and can only be used after being post-amplified.
Due to factors such as signal intensity, photoelectric conversion efficiency of the PIN diode and signal insertion loss, the signal intensities at different optical ports in the add/drop channel might be significantly different with each other. If the gain of the amplification circuit remains constant, strong signals might be saturated or weak signals might be lost, thus the signals are distorted. Therefore, the gain of the amplification circuit should vary with the intensity of the input signal, and it needs to detect the intensity of the signal in each channel. For the multi-channel signal detection circuit, one set of individual detection circuit should be used in each channel in the prior art, and a large number of resistors, capacitors and active devices are used, thus the circuit is relatively complicated.